1. Field of the Invention
The present patent application concerns a new power laser configuration enabling the delivery of a beam of high spatial quality with optimum efficiency of energy extraction. It can be applied more particularly to solid-state Q-switch lasers.
The applications of this type of laser are found notably in telemetry and missile guidance by laser. They make it necessary, for example, to have short pulses (10.sup.-8 s) at a repetition rate of some tens of Hz. The large quantity of energy needed per pulse, from 100 to 300 mJ for example, makes it necessary to develop high peak values, of the order of 10 MW in the example chosen.
Known solid-state Q-switch lasers (YAG, Ruby, GSGG) are used in systems integrating the telemetry or target-illumination functions. Present-day lasers are essentially compact oscillators, flash-lamp-pumped, with triggering by resonant cavity (Q switch). The Q switch is, for example, in the form of an electro-optical crystal with two states: during the switching of the crystal, a giant pulse occurs resulting from the accumulation of energy in the resonant cavity between two switchings.
It is important to be able to increase the output energy of these lasers in order to diminish their divergence, which is presently far from the diffraction limit. One of the reasons, of course, is the need to increase the range of these systems.
Now, the performance characteristics of these flash-lamp-pumped oscillators are limited, all the more so as their mean power increases owing to constraints of a thermal origin induced in the laser rod. These thermal constraints are due to the fact that a major part of the energy delivered by the flash lamps is dissipated in heat in the rod. This results, firstly, in the appearance of a heat lens in the cavity, which induces phase distortions that are difficult to correct in the power build-up mode of the laser and, secondly, by the induction of a birefringency which depolarizes the beam and, hence, leads to a reduction in the energy extracted from the cavity by Q switching.
There are at least two other known approaches that are used to try and reduce these thermal effects, or even to get rid of them:
a first approach consists in reducing the part of energy dissipated in heat in the laser rods. This may be achieved by means of a resonant pumping by power laser diode. Thus, in the article by D.S. Sumida, in IEEE J.QE-24, No. 6, June 1988, it is estimated that the energy dissipated as heat in the YAG crystal laser is reduced by a ratio of 7 to 10 in using diodes rather than flash lamps. It must be noted that the thermal load, although smaller, is not zero and could therefore be substantial if the rate is increased for example. The second approach provides a solution to this constraint.
this second approach consists in dynamically cancelling the effects of the various aberrations. Nonlinear optics provide attractive and efficient solutions to achieve these functions.
The invention falls within the framework of the latter approach.
In a first known embodiment implementing this approach, a MOPA (master-oscillator-power-amplifier) structure laser is used. In this structure, the oscillator has an output energy that is low enough for the beam to have excellent quality (from the viewpoint of its divergence) and the amplifier stage provides the energy to the beam.
However, the drawback encountered in this type of structure is that the generation of aberrations, notably of thermal origin, is transferred to the amplifier stage.
There are known ways of reducing these aberrations very sharply, by compensation as described in the patent document PCT 87/05751 on behalf of the HUGHES Aircraft Company. This document describes a MOPA type laser in which the amplifier stage includes a conjugate mirror positioned on the output optical path of the laser amplifier of the MOPA structure, which reflects the laser ray and prompts a second pass by the beam through the laser amplifier. During the second pass, the radiation that goes through the stage is a radiation in conjugate phase, so as to achieve a compensation of the aberrations. The system described further comprises optical delay means preventing the overlapping of the laser pulses of the first and second passes through the laser amplifier.
The present invention concerns a MOPA-type laser structure working according to principle different from that of the HUGHES system.
More precisely, the invention is aimed at providing a power laser configuration of high efficiency and high spatial quality, without transfer of the aberrations introduced by the power amplifier stage. The idea is to amplify the beam of the oscillator of the MOPA without distortion, in avoiding recourse to specific elements for the compensation of aberrations.